1. Field of Invention
This invention relates to tone-switched telephone systems and particularly to an interface for a telephone switch bus line and a dual tone multiple frequency (DTMF) type tone decoder which inhibits recognition of spurious signals as valid switching tones.
Dual tone multiple frequency (DTMF) signaling has been developed for switching communication channels in telephone communication systems. According to the DTMF technique, a pair of non-harmonically-related tones are transmitted via a signal bus line to a detector which detects the tone pairs and identifies the number associated therewith. The tone pair is made up of one tone from a low frequency group (697, 770, 852, 941 Hz) and one tone from a high frequency group (1209, 1336, 1477, 1633 Hz).
For proper operation, any DTMF detector must recognize a tone pair within a defined bandwidth while tolerating spurious noise, input amplitude variation, and amplitude differential between tones. Further, the detector must operate within timing restrictions imposed by the DTMF generation and detection process as well as meet any other particular requirements of a specific application, such as to distinguish a modulated voice signal from an encoding tone.
DTMF detectors and interface circuitry typically comprise three stages, namely, band splitting filters, an automatic gain control (AGC) and limiter circuit, and a tone decoder. The band splitting filters receive DTMF tone pairs from a telephone bus line and separate the tone pairs into the respective high frequency and low frequency groups and also reject frequency information out of the defined bandwidths which could cause false digits to be decoded. The AGC/limiter stage converts the analog tones into square waves, or constant amplitude pulses. The output of the AGC/limiter stage is processed by a digital tone decoder, which determines the validity of the incoming square wave pulse train as a number tone and converts valid pulse train signals to logic signals for further procession by a switchboard exchange.
Because a tone decoder is a digital device, the interface, particularly the band splitting filters, must have a rejection characteristic of at least 40 dB of signals outside of the pass band. Many filters of this nature are characterized by relatively high susceptibility to transient oscillation at the pass band frequency in response to input signals. As a consequence, any impulses on the input side, such as transients on the signal bus line including contact closure of a telephone key pad, shot noise and the like, can cause the filter circuits to ring for several milliseconds at the pass band frequency of the filters. If the AGC/limiter stage passes the ringing or transient oscillation on to the decoder, undesired errors in the digit decoding process will occur. The result will be incorrectly switched calls and like signaling errors. Accordingly, some provision must be made between the signal bus line input and the tone decoder, particularly in the limiter stage, to eliminate such undesired decoding errors.
2. Description of the Prior Art
Techniques are known to the telephone art for distinguishing extended tone signals from spurious signals. U.S. Pat. No. 3,944,753 discloses a system which includes a frequency drop detector and means for detecting output signals lasting longer than a predetermined period in order to distinguish tone signals from voices and other spurious signals. The '753 patent discloses a system in which the operating threshold of a frequency detection circuit can be varied at different rates in opposite amplitude directions.
U.S. Pat. No. 3,836,727 discloses a discriminator circuit providing a first and second threshold level for use in a DTMF signal detecting scheme. A capacitive charging circuit having a fast attack and slow decay is also disclosed. The '727 patent discloses a detecting scheme wherein both the pure tone signal and the disturbed or noisy tone signal are processed during time intervals of fixed equal length.
These known references fail to expressly teach a technique for masking out transient oscillations in narrow band bandpass filter output signals in a telephone system tone decoding interface circuit.